The present invention relates to a power transmission system for a full-time four-wheel drive vehicle having a mechanism for absorbing the difference between the speeds of the front and rear wheels.
In a part-time four-wheel drive vehicle, the power transmission system can be selectively converted from the two-wheel driving mode to the four-wheel driving mode by engaging a clutch which is manually operated by a select lever.
The two-wheel drive is selected on paved dry surfaces. The reason is as follows. When the vehicle negotiates corners, the front wheels run through an arc of greater radius than that of the rear wheels and therefore tend to rotate faster than the rear wheels.Such a difference between the speeds the of the front and rear wheels causes the braking of the vehicle, known as "tight corner braking". In order to prevent such a braking phenomenon, a full-time four-wheel drive vehicle, a transmission system of which is automatically changed to a two-wheel drive transmission system at a large steering angle, is disclosed in Japanese patent Laid Open No. 57-15019.
Further, Japanese Utility Model Laid Open No. 58-136829 discloses a full-time four-wheel drive tractor having a transmission system in which a one-way clutch (overrunning clutch) is provided in the power transmission for the front wheels. In a normal driving state, the one-way clutch does not engage, thereby establishing the two-wheel driving mode by the rear wheels. When the rear wheels skid, the one-way clutch engages to provide the four-wheel driving mode. However, the system can not prevent the tight corner braking at sharp corners, since the front wheels run faster than the rear wheels, thereby engaging the clutch.